Rhythmic motor acts, such as walking and breathing, are programmed by motor pattern generators which are composed of groups of synaptically interconnected central neurons. The actual neural circuits that comprise these motor pattern generators are known for only a few simple behaviors. Often rhythmic motor acts occur as variations upon the same theme e.g., forward vs. backward walking. It has been hypothesized that these variations are controlled by a single motor pattern generator which can function in alternate metastable coordination states. However, direct evidence for this hypothesis can be obtained only from a well characterized central motor pattern generator which displays metastable coordination. The motor pattern generator controlling heartbeat in the leech is now well characterized in terms of the identified neurons and their synaptic interactions that comprise the pattern generating circuit. Moreover, this motor pattern generator displays metastable coordination associated with the peristaltic and non-peristaltic coordination modes of the paired heart tubes. I propose to further characterize the leech heartbeat motor pattern generator using intracellular recording, stimulating and staining techniques. In particular I plan to complete the analysis of the neuronal mechanisms which underlie metastable coordination and the transitions from one metastable coordination state to another, in this system. Such experiments should shed light on similar phenomena occurring in the less accessible nervous systems of vertebrates.